2.4 Water

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Main Content

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1. Water as a Polar Molecule

Water has the chemical formula H₂O — one oxygen (O) atom bonded to two hydrogen (H) atoms.

The oxygen and hydrogen atoms are held together by covalent bonds (strong chemical bonds formed by sharing electrons). However, the sharing of electrons is not equal — the oxygen atom pulls the shared electrons much more strongly towards itself than the hydrogen atoms do.

This unequal sharing creates a separation of charge within the molecule:

• The oxygen atom has slightly more electrons around it → it gains a small negative charge, written as δ− (say: "delta minus")
• The hydrogen atoms have slightly fewer electrons → they gain a small positive charge, written as δ+ (say: "delta plus")

Because one part of the molecule is slightly negative and another part is slightly positive, water is described as a polar molecule — a molecule with two oppositely charged ends (like a tiny magnet with a positive and negative pole).

The water molecule also has a bent shape (not straight), which means the charges do not cancel each other out — this is what makes it polar.

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2. How Hydrogen Bonds Form Between Water Molecules

Because water is polar, neighbouring water molecules are attracted to each other. Here is exactly how this works:

• The δ+ hydrogen of one water molecule is attracted to the δ− oxygen of a neighbouring water molecule.
• This attraction between them is called a hydrogen bond.

Key points about hydrogen bonds:

• They are represented in diagrams by dotted lines (to show they are weaker than covalent bonds, which are shown as solid lines).
• Each hydrogen bond is individually weak — it breaks and reforms very easily.
• However, in a sample of water there are enormous numbers of hydrogen bonds all at once, so together they create a strong, stable structure.
• One oxygen atom can form a maximum of two hydrogen bonds with neighbouring molecules.